1. Field of the Invention
The present invention relates to a variable capacitor, the capacitance of which can be set by application thereacross of a voltage capable of deforming one of its electrodes.
2. Discussion of the Related Art
FIGS. 1A and 1B are cross-section views of an example of a known variable capacitor in two operating states. This variable capacitor is described in a document of Epoch Technologies under the title “A Zipper-Action Differential Micro-Mechanical Tunable Capacitor”. A lower electrode 1 covered with a dielectric layer 2 is laid on an insulating substrate 3. A pad 4 is laid on the substrate next to conductive layer 1. An upper electrode 5 bearing on one side on pad 4 is placed to overhang dielectric layer 2. Electrodes 1 and 5 of the capacitor are connected to a control circuit that applies a voltage between the electrodes.
At rest, when no voltage is applied between the electrodes, upper electrode 5 is substantially horizontal and parallel to the lower electrode, as illustrated in FIG. 1A. In the activation state, when a voltage is applied between the electrodes, upper electrode 5 deforms on the side non-attached to pad 4, to come in contact with dielectric layer 2, as illustrated in FIG. 1B. Once the upper electrode is in contact, the portion of the upper electrode located close to the contact point is close to the lower electrode and it is possible to bring it to a contact with a low voltage. The upper electrode thus deforms until the mechanical pull-back force becomes equal to the electrostatic force exerted between electrodes.
A disadvantage of such a variable capacitor structure is that it is necessary to apply a strong initial voltage to deform the upper electrode until it contacts dielectric layer 2.
The abstract of a Japanese patent 08/213282 (Patent abstracts of Japan, vol. 1996, N° 12) describes a variable capacitor structure comprising a plane electrode lying on the bottom of a cavity and a movable electrode which is placed above the cavity and bears against the edges of the cavity. The application of a voltage between the electrodes leads to the deformation of the above electrode and consequently to an increase of the capacitance of the capacitor. A disadvantage of this structure is that it is necessary to apply a strong voltage to deform the upper electrode.
Moreover, German patent application DE 43 02 204, and the abstract of Japanese patent 10/154456 (Patent abstracts of Japan, vol. 1998, N° 11) describe switch structures comprising a lower electrode on a slant face of a substrate and a upper electrode formed of a beam overhanging over the substrate from the top of the slant face. Two conductive pads are placed at the extremity of the beam and on the substrate at the bottom of the slant face. The application of a voltage between both electrodes deforms the upper electrode by coming closer to the lower electrode until contacting both pads. These switches work in a “binary” way. The electrodes are either apart either in contact and consequently, these both electrodes do not constitute a capacitor whose capacitance can be adjusted.